In the field of power tools, and specifically in the field of battery-powered hand tools such as drills, two types of motors are known and used—a self-contained “can-type” or “can” motor and an “open frame” motor.
Can motors typically include a housing in the form of a cylindrical can in which essentially all of the components of the motor are received, including the armature, magnets, brushes, motor housing, fan, and bearings. Most DC battery-powered tools incorporate a can motor to drive the power tool output due to the simplicity of assembly and low cost of can motors. Can motors are often preassembled, thereby simplifying assembly of the can motors in power tools. Because the motor components are disposed within a can, a can motor can be operated without the motor being mounted in a product. Typically, can motors include several mounting holes on the front of the can to facilitate attachment to a product or gear train. Such mounting holes on the front of the can simplifies alignment of the motor shaft with the product or gear train with which the can motor is used.
Though assembly of can motors into a power tool is oftentimes fairly simple and cost efficient, the power tool housing must be sized to accommodate the can. For example, screw bosses in the power tool housing must be located outside of the outer periphery of the can, which often creates dead or unused space between the tool housing and the motor, often imposing limitations on the size of the power tool. The larger the power tool, the more difficult it is for some users to operate. For example, increasing the length of a drill also increases the distance of the drill's center of mass from the operator's hand, requiring more work from the operator to maneuver the drill, potentially leading to premature fatigue.
Can motors also often restrict the dimensions of the fan used to cool the motor because the outer diameter of the fan is limited by the inner diameter of the can. In addition, air circulation within the can is oftentimes limited. The combination of the limited fan size and limited air circulation in the can often decreases the cooling capability of the can motor.
Open frame motors are often used in power tools, but typically at a greater cost and greater difficulty of manufacturing compared to can motors. Open frame motors typically include an armature and a permanent magnet rolled flux ring field. Open frame motors must be installed within a power tool prior to their operation because the individual components of the motor are not contained within a common housing inside the tool housing as in a can motor. Open frame motors typically contain fewer components than can motors (specifically, the can or can housing is omitted). The components of an open frame motor are often installed in tool housings individually, often leading to difficulty associated with controlling the component centerline relationships and attachment of the bearings, gear train, and permanent magnet field to the power tool housing. As a result, tool housings incorporating open frame motors must often be manufactured using closer or tighter tolerances.